1. Field of the Invention
This invention relates to solid state light sources that use wavelength conversion. More particularly, it relates to such solid state light sources with high brightness, useful in applications such as projectors.
2. Description of the Related Art
Conventional light source used in projectors is UHP lamp, where mercury plays an important role. More environmental-friendly technologies are desired. One type of solid state light source uses a wavelength conversion scheme to convert an excitation light into a converted light by a wavelength conversion material such as phosphors. U.S. Pat. No. 7,547,114 describes a light source structure, shown in FIG. 1 (adopted from FIG. 1 of the patent), where a solid-state light source 100 is used to provide an excitation light through a focusing system 102, and several color phosphors coated in different segments of a rotating phosphor wheel 104 are excited by the excitation light to generate a light having a color light sequence. In FIG. 1, A is the rotation axis of the color wheel 104.
When the excitation light is a narrow beam light, for example, when the excitation light beam has a divergence angle smaller than 45 degrees, a filter can be used to enhance the luminescent efficacy by recycling the reflected excitation light from the phosphor layer, as shown in FIG. 2. In this structure, the filter 22 (referred to as an angle selective filter) is designed to be transmissive when the excitation light has a small incident angle on the filter (input angle), and reflective when the excitation light has a large incident angle. The excitation light 24A is emitted from the light source 20, and transmitted through the filter 22 with a small incident angle (almost perpendicular to the filter) to illuminate the phosphor film 26. Some of the excitation light is absorbed by the phosphor and converted to luminescence light (converted light) 28A, but some of the excitation light 24B is reflected by the phosphor film 26 with a Lambertian distribution. Most of reflected light 24B, which has large incident angles, is reflected by the filter 22 and goes back into phosphor film 26 to generate a secondary luminescence light 28B. This enhances the recycling of the excitation light, which increases the luminescence efficiency of the entire device. There are many well-know method to convert the excitation lights into a narrow beam. For example, by using a collimation lens, a divergence beam can be converted to a beam with small divergence angle. In addition, there are light sources, such as lasers, that intrinsically generate narrow light beams. For example, blue laser often has a full beam divergence angle smaller than 50 degree. By using a collection lens, the beam divergence angle can be further reduced to less than 10 degree. The method mentioned above works particularly well for laser phosphor devices.
Increasing the excitation light power by coupling additional excitation light source, generally having the same color as the first excitation light source, is another way to enhance the luminescent brightness, as shown in FIG. 3. For example, a blue laser or blue laser array 30 can be used as the primary excitation light source to excite phosphors or other wavelength conversion materials like quantum dots, and a blue LED or LED array 20 can be added as the second excitation light source because of its lower cost compared to blue lasers. There are many methods for coupling the lights from the laser source 20 and the LED source 30. In the illustrated example, a small mirror 36 is used. Because the etendue of the LED source 30 is typically much larger than that of the laser 20, to maximize the LED power, a condenser lens 32 is used to collect and focus the LED light within the full hemisphere.